A typical high-performance vibrating beam accelerometer (VBA) includes dual force-sensitive resonators operated in a push-pull mode to minimize the effects of common mode errors. Such devices have been constructed in basically two different configurations. In the first configuration, two pendulous proof masses are arranged generally parallel to each other in separate, closed cavities. Each proof mass is provided with a quartz crystal resonator to sense the force developed as a result of acceleration directed along a sensitive axis of the accelerometer, e.g., in a direction transverse to the parallel longitudinal axes of the two proof masses.
One benefit of this first configuration is minimization of potential interaction between the two proof masses so that thermal stresses are limited by the stiffness of the flexures and not by the stiffness of the force-sensitive resonators. The resonators in a VBA are inherently very stiff, giving a wide useful bandwidth, but pendulous geometry permits the flexures, which attach the proof mass to the supporting case, to be very soft. Both thermal stress in the resonator and the effects of stress in the accelerometer case can be minimized in this design. Furthermore, the design of this type of dual VBA is relatively straightforward and simple, yielding excellent static performance and long-term stability. However, there are also disadvantages to this configuration. Minor mismatches in damping and in natural resonant frequency between the two proof masses can cause dynamic errors to arise when the accelerometer is subjected to vibration at frequencies approaching the natural resonant frequency of the proof masses. Under such dynamic conditions, the two proof masses may fail to uniformly track the vibration, resulting in a significant undesirable impact on common mode rejection.
The other basic configuration for a VBA uses a single proof mass connected to two quartz crystal resonators. The single proof mass configuration, while appearing simpler than the dual VBA, actually requires much more design expertise and must be mechanically more elaborate to alleviate thermal stress errors that can result from the back-to-back mounting of two quartz crystal resonators to a single proof mass. Typically, the single proof mass configuration provides from five to ten times better dynamic tracking than the conventional dual VBA configuration--but at a price. To resist flexure buckling under transverse loading, the single proof mass generally must be mounted with a flexure that is relatively stiffer in translation than is desirable, causing it to be more sensitive to case stress. For this and other reasons, the single proof mass configuration for a VBA represents a compromise between dynamic performance, complexity, and cost.
Accordingly, it is an object of the present invention to improve the dynamic performance of a dual VBA by minimizing the effect on common mode tracking resulting from operation in an environment where the accelerometer is subjected to vibration close to the resonant frequency of its proof masses. It is a further object of this invention to viscously couple the dual proof masses of the accelerometer with a fluid that damps their non-synchronous vibration. These and other objects and advantages of the invention should be apparent from the attached drawings and the Description of the Preferred Embodiment that follows.